Silicon-waveguide-based broadband polarization splitter-rotator

ABSTRACT

A waveguide-based polarization splitter-rotator (PSR) includes a converter with tapered rib-structure configured to convert TM0/TE0 polarization mode of an input light to a TE1/TE0 mode, a splitter coupled to the first plane for splitting the input light evenly to a first wave at a first port and a second wave at a second port. Furthermore, the PSR includes a phase shifter having a first arm coupled to first port and a second arm coupled to the second port. The first arm guides the first wave to a third port with no phase shift while the second arm adds 90 or 270 degrees to the second wave. The PSR also includes a 2×2 MMI coupler for coupling the first wave and the second wave to output a first output light in TE0 mode exclusively from TM0 mode and a second output light in TE0 mode exclusively from TE0 mode.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of and claims priority to U.S.patent application Ser. No. 15/472,156, filed Mar. 28, 2017, allcommonly assigned and hereby incorporated by references for allpurposes.

BACKGROUND OF THE INVENTION

The present invention relates to a broadband communication device. Moreparticularly, the present invention provides a Si-waveguide-basedbroadband polarization splitter-rotator with low loss and highextinction ratio for polarization-independent silicon photonicscommunication systems.

Over the last few decades, the use of broadband communication networksexploded. In the early days Internet, popular applications were limitedto emails, bulletin board, and mostly informational and text-based webpage surfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Silicon photonics has become very popular for these applications becauseof the potential to combine high performance with low-cost fabrication.In addition, polarization multiplexing is another attractive, low-cost,and simple way to increase transmission capacity. Polarizationsplitter-rotator (PSR) is a key element for polarization management innext generation polarization-independent silicon photonics circuits. Apolarization splitter-rotator preferred for photonic integrated circuits(PICs) should simultaneously have features like compact size, highextinction ratio, low insertion loss, broadband range, stability, simplestructure and high tolerances in manufacture. Conventional polarizationsplitter-rotator is either wavelength sensitive (not suit for broadbandoperation) or based on prism (hard to be made in super compact size).

Therefore, it is desired to develop improved compact polarizationsplitter-rotator that is low insertion loss and wavelength insensitiveacross entire C-band or O-band window for the integrated siliconphotonics circuits.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to photonic broadband communicationdevice. More particularly, the present invention provides a broadbandpolarization splitter-rotator. Merely by example, the present inventiondiscloses a compact polarization splitter-rotator configured as allsilicon waveguide structures for converting a light mixed with both TMmode and TE mode to split to two lights of TE mode respectively at twooutput ports. The compact polarization splitter-rotator can beintegrated in Si photonics circuits for broadbandpolarization-independent communication system, though other applicationsin other wavelength are possible.

In modern electrical interconnect systems, high-speed serial links havereplaced parallel data buses, and serial link speed is rapidlyincreasing due to the evolution of CMOS technology. Internet bandwidthdoubles almost every two years following Moore's Law. But Moore's Law iscoming to an end in the next decade. Standard CMOS silicon transistorswill stop scaling around 5 nm. And the internet bandwidth increasing dueto process scaling will plateau. But Internet and mobile applicationscontinuously demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. This disclosure describestechniques and methods to improve the communication bandwidth beyondMoore's law.

In an embodiment, the present invention provides a polarizationsplitter-rotator for broadband operation. The polarizationsplitter-rotator includes a converter comprising a rib structurewaveguide with symmetrically tapered shapes extended in a lengthwisedirection from an input port to a first cross-section plane. Theconverter is configured to guide an input light with mixed TransverseMagnetic (TM0) polarization mode and Transverse Electric (TE0)polarization mode from the input port to the first cross-section planewith the TM0 mode being coupled to the first order Transverse Electric(TE1) mode and the TE0 mode remained as the zero order TE0 mode.Further, the polarization splitter-rotator includes a splittercomprising a planar waveguide extended further in the lengthwisedirection from the first cross-section plane to a second cross-sectionplane having a first port and a second port separated from each other.The splitter is configured to split the input light substantially evenlyin power to a first wave at the first port and a second wave at thesecond port. Additionally, the polarization splitter-rotator includes aphase-shifter comprising a first waveguide arm coupled to the first portand a second waveguide arm coupled to the second port. The firstwaveguide arm is extended in the lengthwise direction from the firstport to a third port of a third cross-section plane and configured tokeep the first wave at the third port in-phase relative to that at thefirst port. The second waveguide arm is separately extended in thelengthwise direction from the second port to a fourth port of the thirdcross-section plane and configured to add a phase shift to the secondwave at the fourth port relative to that at the second port.Furthermore, the polarization splitter-rotator includes a 2×2 MultimodeInterference (MMI) coupler extended further in the lengthwise directionfrom the third cross-section plane to an output plane having a firstoutput port and a second output port disposed respectively in barposition relative to the third port and the fourth port. The 2×2 MMIcoupler is configured to separately output a first output light in TE0mode substantially originated from the input light in TE0 mode and asecond output light in TE0 mode substantially originated from the inputlight in TM0 mode.

In an alternative embodiment, the present invention provides anintegrated silicon-photonics polarization-division transceivercomprising a polarization splitter-rotator in either its transmitteroutput path or receiver input path to handling light wave of wavelengthsin certain range of O-band. Each polarization splitter-rotator includesa converter waveguide having a rib structure with symmetrically taperedshapes extended in a lengthwise direction from an input port to a firstcross-section plane. The converter waveguide is configured to guide aninput light with mixed Transverse Magnetic (TM0) polarization mode andTransverse Electric (TE0) polarization mode from the input port to thefirst cross-section plane with the TM0 mode being coupled to a firstorder Transverse Electric (TE1) mode and the TE0 mode being remained asa zero order TE0 mode. Further, the polarization splitter-rotatorincludes a splitter comprising a planar waveguide extended further inthe lengthwise direction from the first cross-section plane to a secondcross-section plane having a first port and a second port separated fromeach other. The splitter is configured to split the input lightsubstantially evenly in power to a first wave at the first port and asecond wave at the second port. Additionally, the polarizationsplitter-rotator includes a phase-shifter comprising a first waveguidearm coupled to the first port and a second waveguide arm coupled to thesecond port. The first waveguide arm is extended in the lengthwisedirection from the first port to a third port of a third cross-sectionplane and configured to keep the first wave at the third port in-phaserelative to that at the first port. The second waveguide arm isseparately extended in the lengthwise direction from the second port toa fourth port of the third cross-section plane and configured to add aphase shift of (½)π or (3/2)π to the second wave at the fourth portrelative to that at the second port. Furthermore, the polarizationsplitter-rotator includes a 2×2 Multimode Interference (MMI) couplerextended further in the lengthwise direction from the thirdcross-section plane to an output plane having a first output port and asecond output port disposed respectively in bar position relative to thethird port and the fourth port. The 2×2 MMI coupler is configured toseparately output a first output light in TE0 mode substantiallyoriginated from the input light in TE0 mode and a second output light inTE0 mode substantially originated from the input light in TM0 mode.

Optionally, the first output light in TE0 mode just suffers atransmission loss less than 1.7 dB relative to the input light in TM0mode with wavelengths in a broad range of 1260 nm-1340 nm and the secondoutput light in TE0 mode merely suffers a transmission loss less than1.5 dB relative to the input light in TE0 mode with wavelengths in thesame range of 1260 nm-1340 nm. The first output light and the secondoutput light are respectively outputted via the first output port andthe second output port with an extinction ratio no smaller than 16 dB.

In another alternative embodiment, the present invention provides anintegrated silicon-photonics polarization-division transceivercomprising a polarization splitter-rotator in either its transmitteroutput path or receiver input path to handling light wave of wavelengthsin certain range of C-band. Each polarization splitter-rotator includesa converter waveguide having a rib structure with symmetrically taperedshapes extended in a lengthwise direction from an input port to a firstcross-section plane. The converter waveguide is configured to guide aninput light with mixed Transverse Magnetic (TM0) polarization mode andTransverse Electric (TE0) polarization mode from the input port to thefirst cross-section plane with the TM0 mode being coupled to a firstorder Transverse Electric (TE1) mode and the TE0 mode being remained asa zero order TE0 mode. Further, the polarization splitter-rotatorincludes a splitter comprising a planar waveguide extended further inthe lengthwise direction from the first cross-section plane to a secondcross-section plane having a first port and a second port separated fromeach other. The splitter is configured to split the input lightsubstantially evenly in power to a first wave at the first port and asecond wave at the second port. Additionally, the polarizationsplitter-rotator includes a phase-shifter comprising a first waveguidearm coupled to the first port and a second waveguide arm coupled to thesecond port. The first waveguide arm is extended in the lengthwisedirection from the first port to a third port of a third cross-sectionplane and configured to keep the first wave at the third port in-phaserelative to that at the first port. The second waveguide arm isseparately extended in the lengthwise direction from the second port toa fourth port of the third cross-section plane and configured to add aphase shift of (½)π or (3/2)π to the second wave at the fourth portrelative to that at the second port. Furthermore, the polarizationsplitter-rotator includes a 2×2 Multimode Interference (MMI) couplerextended further in the lengthwise direction from the thirdcross-section plane to an output plane having a first output port and asecond output port disposed respectively in bar position relative to thethird port and the fourth port. The 2×2 MMI coupler is configured toseparately output a first output light in TE0 mode substantiallyoriginated from the input light in TE0 mode and a second output light inTE0 mode substantially originated from the input light in TM0 mode.

Optionally, the first output light in TE0 mode just suffers atransmission loss less than 1.3 dB relative to the input light in TE0mode with wavelengths in a range of 1525 nm-1565 nm and the secondoutput light in TE0 mode just suffers a transmission loss less than 1.4dB relative to the input light in TM0 mode with wavelengths in the samerange of 1525 nm-1565 nm. The first output light and the second outputlight are respectively outputted via the second output port and thefirst output port with an extinction ratio no smaller than 19 dB.

In yet another embodiment, the present invention provides apolarization-independent silicon photonics communication systemcomprising an integrated polarization-division transceiver coupled to apolarization splitter-rotator in either a transmitter output path or areceiver input path. The polarization splitter-rotator includes aconverter comprising a rib structure waveguide with symmetricallytapered shapes extended in a lengthwise direction from an input port toa first cross-section plane. The converter is configured to guide aninput light with mixed Transverse Magnetic (TM0) polarization mode andTransverse Electric (TE0) polarization mode from the input port to thefirst cross-section plane with the TM0 mode being converted to a firstorder Transverse Electric (TE1) mode and the TE0 mode remained as a zeroorder TE0 mode. Further, the polarization splitter-rotator includes asplitter comprising a planar waveguide extended further in thelengthwise direction from the first cross-section plane to a secondcross-section plane having a first port and a second port separated fromeach other. The splitter is configured to split the input lightsubstantially evenly in power to a first wave at the first port and asecond wave at the second port. Additionally, the polarizationsplitter-rotator includes a phase-shifter comprising a first waveguidearm coupled to the first port and a second waveguide arm coupled to thesecond port. The first waveguide arm is extended in the lengthwisedirection from the first port to a third port of a third cross-sectionplane and configured to keep the first wave at the third port in-phaserelative to that at the first port. The second waveguide arm isseparately extended in the lengthwise direction from the second port toa fourth port of the third cross-section plane and configured to add aphase shift to the second wave at the fourth port relative to that atthe second port. Furthermore, the polarization splitter-rotator includesa 2×2 Multimode Interference (MMI) coupler extended further in thelengthwise direction from the third cross-section plane to an outputplane having a first output port and a second output port disposedrespectively in bar position relative to the third port and the fourthport, and configured to separately output a first output light in TE0mode substantially originated from the input light in TE0 mode and asecond output light in TE0 mode substantially originated from the inputlight in TM0 mode. Optionally, as the second waveguide arm of the phaseshifter is configured to add (½)π to the second wave, the first outputlight in TE0 mode suffers a transmission loss less than 1.7 dB relativeto the input light in TM0 mode for a broad O-band wavelengths between1260 nm and 1340 nm or C-band wavelengths between 1525 nm and 1565 nm,and the second output light in TE0 mode also just suffers less than 1.7dB insertion loss relative to input light in TE0 mode. Alternatively, asthe second waveguide arm of the phase shifter is configured to add(3/2)π to the second wave, the first output light in TE0 mode suffers atransmission loss less than 1.7 dB relative to the input light in TE0mode for a broad O-band wavelengths between 1260 nm and 1340 nm orC-band wavelengths between 1525 nm and 1565 nm, and the second outputlight in TE0 mode also just suffers less than 1.7 dB insertion lossrelative to input light in TM0 mode. The first output port and thesecond output port are associated with an extinction ratio no smallerthan 16 dB for TE-to-TE split-conversion versus TM-to-TEsplit-conversion.

Many benefits associated with a polarization-independent siliconphotonics communication system can be achieved with this compactpolarization splitter-rotator in the present disclosure. Thepolarization splitter-rotator is a key element for polarizationmanagement in next-generation integrated silicon photonics systems. Thepolarization splitter-rotator in the current disclosure is compact insize with simple structure adaptive to convenient manufacture process,and highly tolerant in process variation conducted onsilicon-on-insulator (SOI) substrate, which are all crucial for theintegration of photonics communication systems. The polarizationsplitter-rotator in the current disclosure is characterized by a robustSi-waveguide design with less than 1.7 dB transmission loss and greaterthan 16 dB extinction ratio for splitting TM mode and TE mode to twoseparate output ports with both being converted to TE mode. The combinedmode-splitting and mode-rotation functions of the PSR are substantiallyinsensitive to wavelengths over at least 40 nm in O-band or C-band.

The present invention achieves these benefits and others in the contextof known polarization transmitting/receiving technology. However, afurther understanding of the nature and advantages of the presentinvention may be realized by reference to the latter portions of thespecification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1A is a top-view diagram of a waveguide-based polarizationsplitter-rotator according to an embodiment of the present invention.

FIG. 1B is a cross-section view along AA′ plane of the waveguide-basedpolarization splitter-rotator of FIG. 1A according to an embodiment ofthe present invention.

FIG. 1C is a cross-section view along BB′ plane of the waveguide-basedpolarization splitter-rotator of FIG. 1A according to an embodiment ofthe present invention.

FIG. 1D is a cross-section view along CC′ plane of the waveguide-basedpolarization splitter-rotator of FIG. 1A according to an embodiment ofthe present invention.

FIG. 2 is a schematic diagram showing functions of the polarizationsplitter-rotator for handling an input light in TM0 mode and generatingan output light in TE0 mode to one output port according to anembodiment of the present invention.

FIG. 3 is a schematic diagram showing functions of the polarizationsplitter-rotator for handling an input light in TE0 mode and generatingan output light in TE0 mode to a separate output port according to anembodiment of the present invention.

FIGS. 4A and 4B are an exemplary diagram showing intensity distributionsof an input light in TM0 mode passing forward through a polarizationsplitter-rotator to one of two output ports as an output light in TE0mode and an exemplary diagram showing intensity distributions of aninput light in TE0 mode passing forward through the polarizationsplitter-rotator to another output light in TE0 mode, respectively,according to an embodiment of the present invention.

FIG. 5 is a plot of transmission losses of the polarizationsplitter-rotator versus O-band wavelengths according to an embodiment ofthe present invention.

FIG. 6 is a top-view diagram of a waveguide-based polarizationsplitter-rotator according to another embodiment of the presentinvention.

FIGS. 7A and 7B are an exemplary diagram showing intensity distributionsof an input light in TM0 mode passing forward through a polarizationsplitter-rotator to one of two output ports as an output light in TE0mode and an exemplary diagram showing intensity distributions of aninput light in TE0 mode passing forward through the polarizationsplitter-rotator to another output light in TE0 mode according toanother embodiment of the present invention.

FIG. 8 is a plot of transmission losses of the polarizationsplitter-rotator versus C-band wavelengths according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to photonic broadband communicationdevice. More particularly, the present invention provides a broadbandpolarization splitter-rotator. Merely by example, the present inventiondiscloses a compact polarization splitter-rotator configured with allsilicon waveguide structures convert a beam mixed with both TM0 mode andTE0 mode to split to two beams of TE0 mode respectively at two outputports. The compact polarization splitter-rotator can be integrated in Siphotonics circuits for broadband polarization-independent communicationsystem, though other applications in other wavelength and are possible.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIG. 1A is a top-view diagram of a waveguide-based polarizationsplitter-rotator according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. Referring to FIG. 1A atop view of waveguide-based polarization splitter-rotator (PSR) 10 isshown. In some embodiments, the PSR 10 includes a converter, a 50:50splitter 112, a phase shifter, and a 2×2 multimode interference (MMI)coupler 132 that are formed in a monolithically silicon planar waveguideby directly patterning a silicon layer of a silicon-on-insulator (SOI)substrate. In a specific embodiment, a standard platform of the SOIsubstrate comprises a 220 nm Si layer 112 over an oxide layer 1001, forexample as shown as FIG. 1C in cross-sectional view.

In an embodiment, the converter, aiming to provide a desiredpolarization-mode conversion function for an optical wave inputted viathe input port, includes a rib structure waveguide with symmetricallytapered shapes along a lengthwise direction in two segments. The ribstructure waveguide is characterized by a top-layer 102 overlying abottom-layer 101 extended in the lengthwise direction through a firstsegment of a length L₁ from the input port 100 to a joint plane AA′ anda second segment of a length L₂ from the joint plane to the firstcross-section plane 110. The top-layer 102 is narrower than thebottom-layer 101 and both vary throughout the first length L₁ andthroughout the second length L₂ except they have a first common width W₀at the input port 100 and a second common width W₁ at the firstcross-section plane 110. The specific length-width combination of boththe top-layer 102 and the bottom-layer 101 is configured to provide apolarization-mode conversion function for the optical wave transmittedthrough, depending on wavelength ranges of the optical wave. Inparticular, it is desired to have a length-width combination of the ribstructure waveguide capable of achieving a conversion of TransverseMagnetic (TM0) polarization mode to Transverse Electric (TE0)polarization mode while still maintaining TE0 mode as TE0 mode.

FIG. 1B is a cross-section view along the joint plane AA′ of FIG. 1Aaccording to an embodiment of the present invention. Referring to FIG.1A and FIG. 1B, the rib structure is laid and patterned in a SOIsubstrate with a standard 220 nm thick silicon layer. The top-layer 102of a thickness of h_(t) is formed overlying the bottom-layer 101 of athickness h_(b) in an overlay process after the silicon layer of thethickness of h=h_(t)+h_(b)=220 nm over an oxide layer 1001 is patternedfor the rib structure waveguide as part of a monolithic process offorming the PSR 10. In a specific embodiment, the converter isconfigured for handling optical wave of broadband wavelengths in a rangeof 1260 nm to 1340 nm of O-band. In addition, the width W_(t) of thetop-layer 102 at the joint plane is made to be greater than the firstcommon width W₀, the width W_(b) of the bottom-layer 101 at the jointplane is made to be greater than the width W_(t) of the top-layer 102but smaller than the second common width W₁, and the first length L₁ ismade to be shorter than the second length L₂. After fine tuning thelength-width combination (with a standard height of 220 nm) under theabove configuration the rib structure waveguide serves a desiredpolarization mode converter. For an input light with mixed TM0 mode andTE0 mode inputted via the input port 100, the TM0 mode is substantiallyconverted to first-order Transverse Electric (TE1) mode and the TE0 modeis substantially converted to zero-order Transverse Electric (TE0) modeas the input light travels to the first cross-section plane 110.Specifically, the TE1 mode includes two sub-modes, an out-of-phase TE1₁sub-mode and an in-phase TE1₂ sub-mode. The TE0 mode just is a singlephase mode. This conversion function is applicable for all wavelengthsin O-band from about 1260 nm to about 1340 nm.

The same principle is applicable to design a converter for handlingbroadband wavelengths in C-band. FIG. 6 is a top-view diagram of awaveguide-based polarization splitter-rotator configured to C-bandaccording to another embodiment of the present invention. Referring toFIG. 6, PSR 20 includes a converter, a 50:50 splitter, a phase shifter,and a 2×2 MMI coupler that are substantially similar in shape and sizeto those in PSR 10 (of FIG. 1A) and also formed in a monolithicallysilicon planar waveguide by directly patterning a silicon layer of asilicon-on-insulator (SOI) substrate. The converter of PSR 20 issimilarly laid as a rib structure with symmetric tapered shapescharacterized by a top-layer 202 overlying a bottom-layer 201 extendedin the lengthwise direction through a first segment of a length L₁ fromthe input port 200 to a joint plane AA′ and through a second segment ofa length L₂ from the joint plane to the first cross-section plane 210.The top-layer 202 is narrower than the bottom-layer 201, and both varythroughout the first length L₁ and throughout the second length L₂except they have a first common width W₀ at the input port 200 and asecond common width W₁ at the first cross-section plane 210. Yet, due towavelength difference between C-band and O-band, the converter of thePSR 20 is alternatively configured to make the width W_(t) of thetop-layer 202 at the joint plane to be greater than the first commonwidth W₀ but smaller than the second common width W₁, the width W_(b) ofthe bottom-layer 201 at the joint plane to be greater than the widthW_(t) of the top-layer 202 and the second common width W₁, and the firstlength L₁ to be greater than the second length L₂. With thislength-width combination setup for the rib structure waveguide, theconverter is able to convert the TM0 mode of an input light inputted viathe input port 200 to first-order TE1 mode at the first cross-sectionplane 210 and convert the TE0 mode of the input light to zero-order TE0mode at the first cross-section plane 210, for wavelengths in C-bandfrom about 1525 nm to about 1565 nm.

Referring to FIG. 1A, PSR 10 includes a splitter 112 directly coupled tothe first cross-section plane of the converter as part of the monolithicplanar silicon waveguide formed from the 220 nm silicon layer of the SOIsubstrate. FIG. 1C is a cross-section view of the waveguide-based PSR 10along BB′ plane according to an embodiment of the present invention.Referring to FIG. 1C and FIG. 1A, the splitter 112 is a planar waveguidehaving a height h of the 220 nm silicon layer extended in the lengthwisedirection from the first cross-section plane 110 to a secondcross-section plane 120. The first cross-section plane 110 passes theinput light transmitted through the converter. The second cross-sectionplane 120 includes a first port 1201 and a second port 1202 respectivelylocated next to two opposing edges and separated from each other by agap W_(g). In an embodiment, the splitter 112 is designed for splittingthe input light received at the first cross-section plane 110substantially evenly to a first wave at the first port 1201 and a secondwave at the second port 1202.

Referring to FIG. 1A and FIG. 1C, for achieving the 50:50 splittingfunction, the splitter is characterized with symmetrically tapered edgesand extended in the lengthwise direction with increasing widths througha length L₃ starting from the second common width W₁ at the firstcross-section plane 110 to a first maximum width W_(1m), thencontinuously with decreasing widths through a length L₄, and furthercontinuously with decreasing widths through additional length L₅ endedat the second cross-section plane 120. In an embodiment, 50:50 splittingfunction can be particularly set to handle a specific range of broadbandwavelengths, for example, O-band or C-band. Referring to FIG. 1A, thesplitter 112 of PSR 10 is configured to make the length L₃ to be smallerthan the length L₄ and the additional length L₅ even smaller than thelength L₃ with the total length being no greater than 13˜14 μm and thefirst maximum width W_(1m) is no greater than 2.2 μm to handle 50:50power splitting of input light at all wavelengths in O-band from about1260 nm to about 1340 nm no matter what is the polarization modeassociated with the input light at the first cross-section plane 110 andwhat are the relative phase differences in different sub-modes therein.In particular, the splitter 112 with the above length-width combinationis configured to split the input light with the single-phase zero-orderTE0 mode at the first cross-section plane 110 to two waves with in-phaseTE0 sub-modes at the second cross-section plane 120: a first wave withTE0₁ sub-mode is guided to the first port 1201 and a second wave withTE0₂ sub-mode is guided to the second port 1202. At the same time, thesame splitter 112 is configured to split the input light with thefirst-order TE1 mode at the first cross-section plane 110 to two waves,a first wave with the out-of-phase TE1₁ sub-mode and a second wave withthe in-phase TE1₂ sub-mode respectively at the first port 1201 and thesecond port 1202 of the second cross-section plane 120. Since theout-of-phase TE1₁ sub-mode and the in-phase TE1₂ sub-mode inherentlyhave a phase difference of π, the general interference effect within thesplitter 112 is just able to separate the two TE1 sub-modes spatiallywhile maintaining corresponding phases. In particular, the TE1₁ sub-modeis in the first wave guided to the first port 1201 and the TE1₂ sub-modeis in the second wave guided to the second port 1202, while the TE1₁sub-mode retains its phase delay of π relative to the TE1₂ sub-mode, forall wavelengths in O-band from about 1260 nm to about 1340 nm. Note, thegap Wg between the first port 1201 and the second port 1202 is kept tobe no smaller than 0.2 μm to ensure that the waveguide patterningprocess is practically within its tolerance range of process variation.

In a specific embodiment, the length-width combination of the splitterwaveguide must be reconfigured to allow the splitter to handlealternative broadband of wavelengths. Referring to FIG. 6, under thesame principle yet with reconfigured length-width combination, thesplitter 212 of PSR 20, which has substantially similar shape and sameheight h of 220 nm as the splitter 112, is able to handling 50:50splitting of input light of any polarization mode with a broadband ofwavelengths in C-band. In particular, the length L₃ is made to begreater than the length L₄ and yet the length L₅ is set to be zero withtotal length being set no greater than 2 μm and the first maximum widthW_(1m) at the joint plane connecting the length L₃ to the length L₄being set no greater than 1.5 μm. Under such configuration, the splitter212 of PSR 20 is operated to split the input light at any wavelength inC-band from about 1525 nm to about 1565 nm provided at a firstcross-section plane 210 to a first wave to the first port 2201 and asecond wave to the second port 2202 at a second cross-section plane 220.The polarization mode of each of the first wave and the second wave aswell as their relative phase difference are handled the same way as thesplitter 112 of PSR 10.

Referring to FIG. 1A again, the PSR 10 includes a phase shifterwaveguide coupled to or naturally extended from the first port 1201 andthe second port 1202 at the second middle cross-section plane 120. Thephase shifter waveguide includes a first waveguide arm 121 coupled tothe first port 1201 and a second waveguide arm 122 coupled to the secondport 1202, both having the same height h of the 220 nm silicon layer.The first waveguide arm 121 is extended in the lengthwise direction to athird port 1301 of a third cross-section plane 130 and the secondwaveguide arm 122 is separately extended in the lengthwise direction toa fourth port 1302 of the third cross-section plane 130. In anembodiment, the first waveguide arm 121 is configured to receive thefirst wave from the first port 1201 and to transmit the first wavethrough at least a length L₆ towards the third port 1301 while keepingthe first wave at the third port 1301 in-phase relative to that at thefirst port 1201. The second waveguide arm 122 is configured to receivethe second wave from the second port 1202 and to transmit the secondwave through a separate path of the same length L₆ towards the fourthport 1302 while adding a phase shift to the second wave at the fourthport 1302 relative to that at the second port 1202.

FIG. 1D is a cross-section view of the waveguide-based PSR 10 along CC′plane according to an embodiment of the present invention. Referring toFIG. 1A and FIG. 1D, the first waveguide arm 121 of the phase shifterincludes a straight bar shape of at least the length L₆ and a first armwidth W_(1a) connected between the first port 1201 and the third port1301, and the second waveguide arm 122 of the phase shifter includes astraight bar shaped portion having at least the length L₆ joined asidewith a triangle shaped portion connected between the second port 1202and the fourth port 1302. The second waveguide arm 122 has a varyingsecond arm width W_(2a) which increases from the first arm width W_(1a)at one end to a maximum at an apex of the triangle shaped portion thendecreasing again to the first arm width W_(1a) at the other end. Theconstant width W_(1a) associated with the length L₆ in the firstwaveguide arm 121 effectively retains first wave in-phase travellingthrough the first waveguide arm 121 to reach the third port 1301 at thethird cross-section plane 130. At the same time, the varying widthW_(2a) associated with the length L₆ in the second waveguide arm 122 canbe adjusted to provide a desired phase delay to the second wavetraveling independently through the second waveguide arm 122 to reachthe fourth port 1302 at the third cross-section plane 130. In a specificconfiguration, the maximum W_(2a) is set to be slightly smaller thantwice of the first arm width W_(1a) and the length L₆ is no greater than11 μm to cause a phase delay of (½)π in the second wave through thesecond waveguide arm 122. Alternatively, with lightly reduction in themaximum W_(2a) and increase in the length L₆, a phase delay of (3/2)πcan be produced to the second wave through the second waveguide arm 122.In principle, a phase shift of (π/2+nπ) can be generated for n being anyinteger though effective phase values are all limited within 2π. Despiteother phase delay values can be produced, the above two phase delays aredirectly utilized in the PSR 10 (FIG. 1A) and PSR 20 (FIG. 6) of thepresent disclosure for handling light with wavelengths in both O-bandand C-band.

Referring to FIG. 1A again, the PSR 10 includes a 2×2 MMI coupler 132 asa planar waveguide of the same height h of 220 nm silicon layernaturally extended from the third port 1301 and the fourth port 1302 atthe third cross-section plane 130 to an output plane 140 with a firstoutput port 1401 and a second output port 1402. From the input port 100to the fourth cross-section plane 140, the PSR 10 includes a totallength less than 100 μm, thus forming a very compact sized devicesuitable for highly integrated silicon photonics communication system.The 2×2 MMI coupler 132 is characterized by a rectangular shape of alength L₇ measured from the third cross-section plane 130 to the outputplane 140 and a width W₂. The first output port 1401 is aligned with thethird port 1301 in a bar position at a distance W_(p) away from acentral line of the rectangular shaped planar waveguide in thelengthwise direction. The first output port 1401 is in a cross positionrelative to the fourth port 1302. The second output port 1402 and thefourth port 1302 are respectively in mirror symmetric positions relativeto the first output port 1401 and the third port 1301, neverthelessmaking the second output port 1402 to be in a cross-position relative tothe third port 1301. The 2×2 MMI coupler 132 in such configurationinduces a general interference of optical waves coupled via both thethird port 1301 and the fourth port 1302 and outputs a first outputlight in TE0 mode to the first output port 1401 and a second outputlight in TE0 mode to the second output port 1402. Depending on specificpolarization modes and phase difference of the first wave at the thirdport 1301 and the second wave at the fourth port 1302, optionally, thefirst output light may be exclusively originated from the input lightwith TM0 mode and the second output light may be exclusively originatedfrom the input light with TE0 mode. In other words, the PSR 10 is ableto split the input light with mixed TM0 mode and TE0 mode to guide onepart substantially exclusively with a TM0 mode and rotate it to a TE0mode and guide another part substantially exclusively with a TE0 modeand retain it as a TE0 mode, and respectively to output the two partsseparately to two output ports.

Alternatively, referring to FIG. 6, the PSR 20 includes a substantiallysimilar 2×2 MMI coupler 232 as a planar waveguide of the same height hof 220 nm silicon layer naturally extended from the third port 2301 andthe fourth port 2302 at the third cross-section plane 230 to an outputplane 240 with a first output port 2401 and a second output port 2402.The PSR 20 is specifically configured to handling light wavepolarization mode splitting and rotating for wavelengths in C-band,e.g., about 1525 nm to about 1565 nm. From the input port 200 to thefourth cross-section plane 240, the PSR 20 includes a total length thatcan be limited within 50 μm, forming a ultra-compact sized devicesuitable for highly integrated silicon photonics communication system.

FIG. 2 is a schematic diagram showing functions of the polarizationsplitter-rotator for handling an input light in TM0 mode and generatingan output light in TE0 mode to one output port according to anembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. In an embodiment, the polarization splitter-rotator isPSR 10 of FIG. 1A as disclosed earlier in the specification. Referringto FIG. 2, the TM0 mode of the input light, depicted by a narrowwaveform (though its polarization is along a direction perpendicular toa top surface of the PSR 10), is inputted via the input port 100. Theconverter with a rib-structure having a narrower top-layer 102 over awider bottom layer 101 is configured to convert the TM0 mode tofirst-order TE1 mode at the first cross section 110. The TE1 modeincludes a 180-degree out-of-phase sub-mode (with a phase delay of π)depicted by a backward waveform and an in-phase sub-mode depicted by aforward waveform. Further the splitter 112 is configured to couple tothe first cross-section plane 110 to receive the input light with TE1mode and split the input light substantially evenly to a first wave atthe first port 1201 and a second wave at the second port 1202 both atthe second cross-section plane 120. In particular, the first wave bearssubstantially the out-of-phase TE1 sub-mode and the second wave bearssubstantially the in-phase TE1 sub-mode, the out-of-phase TE1 sub-modehas a phase delay of π relative to the in-phase TE1 sub-mode.

Referring to FIG. 2, in addition, the phase shifter having a firstwaveguide arm 121 coupled to the first port 1201 and the secondwaveguide arm 122 coupled to the second port 1202 to respectivelyhandling the first wave and the second wave. The first wave travelsthrough the first waveguide arm 121 to reach the third port 1301,substantially keeping the same TE1 sub-mode in the same phase, i.e., the180-degree out-of-phase TE1 sub-mode, and the second wave travelsthrough the second waveguide arm 122 to reach the fourth port 1302 witha phase shift of +90 degrees, making a phase difference of 270 degreesbetween the first wave and the second wave. In other words, the firstwave bearing a TE1 sub-mode with (3/2)π phase delay relative to thesecond wave bearing a TE1 sub-mode.

Further referring to the FIG. 2, the 2×2 MMI coupler 132 is configuredto induce general interference of the first wave bearing TE1 sub-modeand the second wave bearing TE1 sub-mode respectively received via thethird port 1301 and the fourth port 1302 with a phase difference of 270degrees and to generate an output light bearing a TE0 mode substantiallyexclusively at the first output port 1401.

FIG. 3 is a schematic diagram showing functions of the polarizationsplitter-rotator for handling an input light in TE0 mode and generatingan output light in TE0 mode to a separate output port according to anembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. In an embodiment, the polarization splitter-rotator isPSR 10 of FIG. 1A as disclosed earlier in the specification. As shown,an input light with TE0 mode is inputted via the input port 100 into thePSR 10. The converter retains substantially zero-order TE0 mode at thefirst cross-section plane 110. Then, the splitter 112 splits the inputlight substantially evenly to a first wave at the first port 1201 and asecond wave at the second port 1202 both bearing a TE0 sub-mode at thesecond cross-section plane 120. The phase shifter then performs itsfunction the same way as above, i.e., retaining the polarization modeand phase of the first wave at the third port 1301 after travelingthrough the first waveguide arm 121 and adding a phase shift of 90degrees to the second wave at the fourth port 1302 after travelingthrough the second waveguide arm 122. Thus, a phase difference of 90degrees exists between the first wave bearing TE0 sub-mode at the thirdport 1301 and the second wave bearing TE0 sub-mode at the fourth port1302. The 2×2 MMI coupler 132 is configured to induce generalinterference of the first wave bearing TE0 sub-mode and the second wavebearing TE0 sub-mode respectively received via the third port 1301 andthe fourth port 1302 with a phase difference of 90 degrees, therebygenerating an output light bearing a TE0 mode substantially exclusivelyat the second output port 1402.

FIGS. 4A and 4B are an exemplary diagram showing intensity distributionsof an input light in TM0 mode passing forward through a polarizationsplitter-rotator to one of two output ports as an output light in TE0mode and an exemplary diagram showing intensity distributions of aninput light in TE0 mode passing forward through the polarizationsplitter-rotator to another output light in TE0 mode, respectively,according to an embodiment of the present invention. In an embodiment,the polarization splitter-rotator is PSR 10 of FIG. 1A as amonolithically formed planar waveguide disclosed earlier in thespecification. As shown in FIG. 4A, the PSR receives a TE0 mode lightwave and firstly couples it to a zero-order TE0 mode light wave.Secondly, the PSR splits the zero-order TE0 mode light wave to a firstwave with TE0₁ sub-mode to an upper waveguide branch and a second wavewith TE0₂ sub-mode to a lower waveguide branch. Both the first wave andthe second wave are in-phase to each other. Further down the paths, thePSR is configured to retain the phase of the first wave with TE0₁sub-mode at an end of the upper waveguide branch and to add 90 degreesphase shift to the second wave with TE0₂ sub-mode at an end of the lowerwaveguide branch. Lastly, the PSR couples the first wave and the secondwave with added 90 degrees phase shift to induce a spatial interferencedistribution and generate an output wave with TE0 mode substantiallyexclusively to one output port to complete a TE path. Optionally,depending on the phase difference value of 90 degrees (or π/2) betweenthe first wave and the second wave and their polarization modes (TE0),the one output port is one at a lower position as shown in FIG. 4A andthe TE path of the PSR 10 is from the input port 100 to the secondoutput port 1402 of FIG. 1A.

Alternatively, as shown in FIG. 7A for the PSR 20 of FIG. 6, the PSR isconfigured to retain the phase of the first wave with TE0₁ sub-mode atan end of the upper waveguide branch and adds 270 degrees phase shift tothe second wave with TE0₂ sub-mode at an end of the lower waveguidebranch. Then, the PSR couples the first wave with TE0₁ sub-mode and thesecond wave with TE0₂ sub-mode and added 270 degrees phase shift toinduce a spatial interference distribution and generate an output wavewith TE0 mode substantially exclusively to the first output port. The TEpath of the PSR 20 in this case is from the input port 200 to the firstoutput port 2401 of FIG. 6.

Referring to FIG. 4B, the PSR receives a TM0 mode light wave and firstlycouples it to a first-order TE1 mode light wave. Secondly, the PSRsplits the first-order TE1 mode light wave to a first wave with TE1₁sub-mode to an upper waveguide branch and a second wave with TE1₂sub-mode to a lower waveguide branch. The first wave with TE1₁ sub-modehas a phase delay of 180 degrees (π) relative to the second wave TE1₂sub-mode. Further down the paths, the PSR retains the phase of the firstwave with TE1₁ sub-mode at an end of the upper waveguide branch and adds90 degrees phase shift to the second wave with TE1₂ sub-mode at an endof the lower waveguide branch. Thus, the first wave with TE1₁ sub-modenow has a phase delay of 270 degrees relative to the second wave withTE1₂ sub-mode. Lastly, the PSR couples the first wave with TE1₁ sub-modeand the second wave with TE1₂ sub-mode and 270 degrees (3π/2) phasedifference to induce a spatial interference distribution and generate anoutput wave with TE0 mode substantially exclusively to one output portto complete a TM path. Optionally, depending on the phase differencevalue of 270 degrees (or 3π/2) between the first wave and the secondwave and their polarization modes (TE1), the output port is one at anupper position as shown in FIG. 4B and the TM path of the PSR is fromthe input port to the first output port of FIG. 1A.

Alternatively, as shown in FIG. 7B for PSR 20 of FIG. 6, the PSR isconfigured to retain the phase of the first wave with TE1₁ sub-mode atan end of the upper waveguide branch and adds 270 degrees phase shift tothe second wave with TE1₂ sub-mode at an end of the lower waveguidebranch. Then, the second wave effectively has a total phase shift of180+270 degrees, or effectively 90 degrees phase difference, relative tothe first wave. Lastly, the PSR couples the first wave with TE1₁sub-mode and the second wave with TE1₂ sub-mode and 90 degrees (π/2)phase difference to induce a spatial interference distribution andgenerate an output wave with TE0 mode substantially exclusively to thesecond output port. The TM path of the PSR 20 in this case is from theinput port 200 to the second output port 2402 of FIG. 6. Nevertheless,the TM path of the PSR is substantially exclusive to the TE path of thePSR. Therefore, the input light with mixed TM0 mode and TE0 mode isexclusively split apart and the TM0 mode of the input light is alsorotated to TE0 mode by the PSR of the present disclosure.

In an embodiment, the PSR of the present disclosure under a single fixedconfiguration separate TM0 mode from the TE0 mode of a light wave andconverts the TM0 mode also to TE0 mode substantially insensitive to abroad range of wavelengths, either an O-band or a C-band, which is adesirable feature required for many applications ofpolarization-independent optical communications.

FIG. 5 is a plot of transmission losses of the polarizationsplitter-rotator versus O-band wavelengths according to an embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications. Inan embodiment, the polarization splitter-rotator is the PSR 10 disclosedabove and used for handling a light wave with any wavelengths in O-bandfrom about 1260 nm to about 1340 nm. As shown, curve 1121 shows a resultof a transmission loss variation of the light wave in TE0 mode over theO-band wavelengths passing the PSR 10 with retained TE0 mode (TE0→TE0)through the TE path and curve 1123 shows a result of a transmission lossvariation of the light wave in TM0 mode over the O-band wavelengthspassing the PSR 10 with converted TE0 mode (TM0→TE0) through the TMpath. Both plots are further shown in an expanded y-axis scale (in upperportion of the figure). The transmission loss for TE0 mode light throughTE path is less than 1.1 dB with about 0.4 dB variation across entirerange of wavelengths shown. The transmission loss for TM0 light throughTM path is less than 1.7 dB with about 0.7 dB variation across the samerange of wavelengths. It demonstrates that PSR 10 of the presentdisclosure has high performance property to split and rotate TM0 modeand TE0 mode of the O-band light with very low insertion loss.

In addition, PSR 10 of the present disclosure also is characterized by ahigh extinction ratio for splitting the TM0 mode and the TE0 mode.Referring to FIG. 5, curve 1122 shows a result of power loss of greaterthan 17 dB for light with TE0 mode leaking through the TM path of thePSR 10 and curve 1124 shows a result of power loss of greater than 16 dBfor light with TM0 mode leaking through the TE path of the PSR 10.

FIG. 8 is a plot of transmission losses of the polarizationsplitter-rotator versus C-band wavelengths according to an embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications. Inan embodiment, the polarization splitter-rotator is the PSR 20 disclosedabove and used for handling a light wave with any wavelengths in C-bandfrom about 1525 nm to about 1565 nm. As shown, curve 2121 shows a resultof a transmission loss variation of the light wave in TE0 mode over theC-band wavelengths passing the PSR 20 with retained TE0 mode (TE0→TE0)through the TE path and curve 2124 shows a result of a transmission lossvariation of the light wave in TM0 mode over the C-band wavelengthspassing the PSR 20 with converted TE0 mode (TM0→TE0) through the TMpath. Both plots are further shown in an expanded y-axis scale (in upperportion of the figure). The transmission loss for TE0 mode light throughthe TE path is less than 1.3 dB with about 0.2 dB variation acrossentire range of wavelengths shown. The transmission loss for TM0 lightthrough TM path is less than 1.4 dB with about 0.3 dB variation acrossthe same range of wavelengths. It demonstrates that PSR 20 of thepresent disclosure has high performance property to split and rotate TM0mode and TE0 mode of the C-band light with very low insertion loss.

In addition, PSR 20 of the present disclosure also is characterized by ahigh extinction ratio for splitting the TM0 mode and the TE0 mode.Referring to FIG. 8, curve 2122 shows a result of power loss of greaterthan 21 dB for light with TE0 mode leaking through the TM path of thePSR 20 and curve 2123 shows a result of power loss of greater than 19 dBfor light with TM0 mode leaking through the TE path of the PSR 20.

Accordingly, the present invention provides, inter alia, awaveguide-based polarization splitter-rotator, apolarization-independent device for integrating polarization-divisiondevices with polarization-independent silicon photonics communicationsystem using a SOI substrate with standard 220 nm silicon layer thatsubstantially obviate one or more of the problems due to limitations anddisadvantages of the related art. In one aspect, the present inventionprovides an integrated silicon-photonics polarization-divisiontransceiver comprising a polarization splitter-rotator in either itstransmitter output path or receiver input path to handling light wave ofwavelengths in certain range of O-band. Each polarizationsplitter-rotator includes a converter waveguide having a rib structurewith symmetrically tapered shapes extended in a lengthwise directionfrom an input port to a first cross-section plane. The converterwaveguide is configured to guide an input light with mixed TransverseMagnetic (TM0) polarization mode and Transverse Electric (TE0)polarization mode from the input port to the first cross-section planewith the TM0 mode being converted to a first order Transverse Electric(TE1) mode and the TE0 mode being converted to a zero order TE0 mode.Further, the polarization splitter-rotator includes a splittercomprising a planar waveguide extended further in the lengthwisedirection from the first cross-section plane to a second cross-sectionplane having a first port and a second port separated from each other.The splitter is configured to split the input light substantially evenlyin power to a first wave at the first port and a second wave at thesecond port. Additionally, the polarization splitter-rotator includes aphase-shifter comprising a first waveguide arm coupled to the first portand a second waveguide arm coupled to the second port. The firstwaveguide arm is extended in the lengthwise direction from the firstport to a third port of a third cross-section plane and configured tokeep the first wave at the third port in-phase relative to that at thefirst port. The second waveguide arm is separately extended in thelengthwise direction from the second port to a fourth port of the thirdcross-section plane and configured to add a phase shift of (½)π to thesecond wave at the fourth port relative to that at the second port.Furthermore, the polarization splitter-rotator includes a 2×2 MultimodeInterference (MMI) coupler extended further in the lengthwise directionfrom the third cross-section plane to an output plane having a firstoutput port and a second output port disposed respectively in barposition relative to the third port and the fourth port. The 2×2 MMIcoupler is configured to separately output a first output light in TE0mode substantially originated from the input light in TE0 mode and asecond output light in TE0 mode substantially originated from the inputlight in TM0 mode. Optionally, the first output light in TE0 mode justsuffers a transmission loss less than 1.7 dB relative to the input lightin TM0 mode with wavelengths in a broad range of 1260 nm-1340 nm and thesecond output light in TE0 mode merely suffers a transmission loss lessthan 1.5 dB relative to the input light in TE0 mode with wavelengths inthe same range of 1260 nm-1340 nm. The first output light and the secondoutput light are respectively outputted via the first output port andthe second output port with an extinction ratio no smaller than 16 dB.

In another aspect, the present invention provides an integratedsilicon-photonics polarization-division transceiver comprising apolarization splitter-rotator in either its transmitter output path orreceiver input path to handling light wave of wavelengths in certainrange of C-band. Each polarization splitter-rotator includes a converterwaveguide having a rib structure with symmetrically tapered shapesextended in a lengthwise direction from an input port to a firstcross-section plane. The converter waveguide is configured to guide aninput light with mixed Transverse Magnetic (TM0) polarization mode andTransverse Electric (TE0) polarization mode from the input port to thefirst cross-section plane with the TM0 mode being converted to a firstorder Transverse Electric (TE1) mode and the TE0 mode being converted toa zero order TE0 mode. Further, the polarization splitter-rotatorincludes a splitter comprising a planar waveguide extended further inthe lengthwise direction from the first cross-section plane to a secondcross-section plane having a first port and a second port separated fromeach other. The splitter is configured to split the input lightsubstantially evenly in power to a first wave at the first port and asecond wave at the second port. Additionally, the polarizationsplitter-rotator includes a phase-shifter comprising a first waveguidearm coupled to the first port and a second waveguide arm coupled to thesecond port. The first waveguide arm is extended in the lengthwisedirection from the first port to a third port of a third cross-sectionplane and configured to keep the first wave at the third port in-phaserelative to that at the first port. The second waveguide arm isseparately extended in the lengthwise direction from the second port toa fourth port of the third cross-section plane and configured to add aphase shift of (3/2)π to the second wave at the fourth port relative tothat at the second port. Furthermore, the polarization splitter-rotatorincludes a 2×2 Multimode Interference (MMI) coupler extended further inthe lengthwise direction from the third cross-section plane to an outputplane having a first output port and a second output port disposedrespectively in bar position relative to the third port and the fourthport. The 2×2 MMI coupler is configured to separately output a firstoutput light in TE0 mode substantially originated from the input lightin TE0 mode and a second output light in TE0 mode substantiallyoriginated from the input light in TM0 mode. Optionally, the firstoutput light in TE0 mode just suffers a transmission loss less than 1.3dB relative to the input light in TE0 mode with wavelengths in a rangeof 1525 nm-1565 nm and the second output light in TE0 mode just suffersa transmission loss less than 1.4 dB relative to the input light in TM0mode with wavelengths in the same range of 1525 nm-1565 nm. The firstoutput light and the second output light are respectively outputted viathe second output port and the first output port with an extinctionratio no smaller than 19 dB.

In yet another aspect, the present disclosure additionally provides apolarization-independent silicon photonics communication systemcomprising an integrated polarization-division transceiver coupled to apolarization splitter-rotator in either a transmitter output path or areceiver input path. The polarization splitter-rotator includes aconverter comprising a rib structure waveguide with symmetricallytapered shapes extended in a lengthwise direction from an input port toa first cross-section plane. The converter is configured to guide aninput light with mixed Transverse Magnetic (TM0) polarization mode andTransverse Electric (TE0) polarization mode from the input port to thefirst cross-section plane with the TM0 mode being coupled to a firstorder Transverse Electric (TE1) mode and the TE0 mode being coupled to azero order TE0 mode. Further, the polarization splitter-rotator includesa splitter comprising a planar waveguide extended further in thelengthwise direction from the first cross-section plane to a secondcross-section plane having a first port and a second port separated fromeach other. The splitter is configured to split the input lightsubstantially evenly in power to a first wave at the first port and asecond wave at the second port. Additionally, the polarizationsplitter-rotator includes a phase-shifter comprising a first waveguidearm coupled to the first port and a second waveguide arm coupled to thesecond port. The first waveguide arm is extended in the lengthwisedirection from the first port to a third port of a third cross-sectionplane and configured to keep the first wave at the third port in-phaserelative to that at the first port. The second waveguide arm isseparately extended in the lengthwise direction from the second port toa fourth port of the third cross-section plane and configured to add aphase shift to the second wave at the fourth port relative to that atthe second port. Furthermore, the polarization splitter-rotator includesa 2×2 Multimode Interference (MMI) coupler extended further in thelengthwise direction from the third cross-section plane to an outputplane having a first output port and a second output port disposedrespectively in bar position relative to the third port and the fourthport, and configured to separately output a first output light in TE0mode substantially originated from the input light in TE0 mode and asecond output light in TE0 mode substantially originated from the inputlight in TM0 mode. Optionally, as the second waveguide arm of the phaseshifter is configured to add (½)π to the second wave, the first outputlight in TE0 mode suffers a transmission loss less than 1.7 dB relativeto the input light in TM0 mode for a broad O-band wavelengths between1260 nm and 1340 nm or C-band wavelengths between 1525 nm and 1565 nm,and the second output light in TE0 mode also just suffers less than 1.7dB insertion loss relative to input light in TE0 mode. Alternatively, asthe second waveguide arm of the phase shifter is configured to add(3/2)π to the second wave, the first output light in TE0 mode suffers atransmission loss less than 1.7 dB relative to the input light in TE0mode for a broad O-band wavelengths between 1260 nm and 1340 nm orC-band wavelengths between 1525 nm and 1565 nm, and the second outputlight in TE0 mode also just suffers less than 1.7 dB insertion lossrelative to input light in TM0 mode. The first output port and thesecond output port are associated with an extinction ratio no smallerthan 16 dB for TE-to-TE split-conversion versus TM-to-TEsplit-conversion.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A polarization splitter-rotator for broadbandoperation comprising: a rib structure waveguide with symmetricallytapered shapes including a top-layer overlying a bottom-layer extendedin the lengthwise direction through a first length from an input port toa joint plane and a second length longer than the first length from thejoint plane to the first cross-section plane, wherein the top-layerbeing characterized with increasing widths through the first length froma first common width at the input port to a top width at the joint planeand further with increasing widths through the second length from thetop width to a second common width at the first cross-section plane, thebottom-layer being characterized with varying widths through the firstlength from the first common width at the input port to a bottom widthat the joint plane and further through the second length from the bottomwidth to a second common width; a planar waveguide extended further inthe lengthwise direction from the first cross-section plane to a secondcross-section plane having a first port and a second port separated fromeach other, and configured to split light in O-band into the input portevenly to a first wave at the first port and a second wave at the secondport; a phase-shifter comprising a first waveguide arm connected thefirst port to a third port of a third cross-section plane and a secondwaveguide arm connected the second port to a fourth port of the thirdcross-section plane, the first waveguide arm being configured to keepthe first wave at the third port in-phase relative to that at the firstport, the second waveguide arm being configured to add a phase shift tothe second wave at the fourth port relative to that at the second port;and a 2×2 Multimode Interference (MMI) coupler extended further from thethird cross-section plane to an output plane having a first output portand a second output port disposed respectively in bar position relativeto the third port and the fourth port, and configured to separatelyoutput a first output light in TE mode substantially originated from thelight into the input port in TE mode and a second output light in TEmode substantially originated from the light into the input port in TMmode.
 2. The polarization splitter-rotator of claim 1, wherein the ribstructure waveguide, the planar waveguide, the phase-shifter, and 2×2MMI coupler comprise a monolithic planar body patterned from a siliconlayer of a silicon-on-insulator (SOI) substrate.
 3. The polarizationsplitter-rotator of claim 2, wherein the silicon layer comprises acommon height of proximately 220 nm.
 4. The polarizationsplitter-rotator of claim 1, wherein the light in O-band into the inputport comprises any wavelength from about 1260 nm to about 1340 nm,wherein the second common width is no greater than 1 μm and the secondlength is no greater than 33 μm.
 5. The polarization splitter-rotator ofclaim 1, wherein the planar waveguide is characterized withsymmetrically tapered edges extended in the lengthwise direction withincreasing widths in a third length started from the second common widthat the first cross-section plane to a first maximum width thencontinuously with decreasing widths in a fourth length, and furthercontinuously with decreasing widths in a fifth length ended at thesecond cross-section plane, the first port and the second port beingseparated from each other by a gap no smaller than 0.2 μm.
 6. Thepolarization splitter-rotator of claim 5, wherein the third length issmaller than the fourth length and the fifth length is smaller than thethird length, wherein the fourth length is no greater than 10 μm and thefirst maximum width is no greater than 2.5 μm.
 7. The polarizationsplitter-rotator of claim 1, wherein the first wave at the first portcomprises a first TE1 sub-mode originated from the light into the inputport with the TM mode and a first TE0 sub-mode originated from the lightinto the input port with the TE mode, the second wave at the second portcomprises a second TE1 sub-mode originated from the light into the inputport with the TM mode and a second TE0 sub-mode originated from thelight into the input port with the TE mode, wherein the first wave withthe first TE1 sub-mode is a phase π behind the second wave with thesecond TE1 sub-mode and the first wave with the first TE0 sub-mode isin-phase with the second wave with the second TE0 sub-mode.
 8. Thepolarization splitter-rotator of claim 7, wherein the first waveguidearm of the phase shifter comprises a straight bar shape of a sixthlength and a first arm width, and the second waveguide arm of the phaseshifter comprises a straight bar shaped portion joined aside with atriangle shaped portion along the sixth length and a second arm widthincreasing from the first arm width to a second maximum width near amiddle position of the six length and decreasing again to the first armwidth.
 9. The polarization splitter-rotator of claim 8, wherein thefirst arm width is no greater than 0.5 μm and the second maximum widthis no greater than twice of the first arm width for keeping the firstwave at the third port in-phase relative to that at the first portthrough the first waveguide arm and generating the phase shift of(π/2+nπ) for the second wave relative to that at the second port throughthe second waveguide arm, wherein n is selected from 0 and
 1. 10. Thepolarization splitter-rotator of claim 9, wherein the 2×2 MMI couplerwaveguide comprises a rectangular shape having a seventh length and asecond width configured to couple the first wave with the first TE1sub-mode at the third port with the second wave with the second TE1sub-mode with a phase difference of (3/2)π to produce the first outputlight in TE0 mode at the first output port, and configured to couple thefirst wave with the first TE0 sub-mode at the third port with the secondwave with the second TE0 sub-mode with a phase difference of (½)π toproduce the second output light in TE0 mode at the second output port,wherein n is selected to be 0 in a configuration of the phase shifter.11. The polarization splitter-rotator of claim 10, wherein the seventhlength is about 4 to 5 times larger than the second width for generatingthe first output light in TE mode with a transmission loss less than 1.7dB relative to the light into the input port in TM mode with anywavelength from about 1260 nm to about 1340 nm and generating the secondoutput light in TE0 mode with a transmission loss less than 1.5 dBrelative to the light into the input port in TE mode with any wavelengthfrom about 1260 nm to about 1340 nm, wherein the second output lightincludes −17 dB or smaller portion of the light into the input port inTM mode and the first output light includes −16 dB or smaller portion ofthe light into the input port in TE mode.
 12. The polarizationsplitter-rotator of claim 11, wherein the transmission loss of the firstoutput light relative to the light into the input port in TM mode andthe transmission loss of the second output light relative to the lightinto the input port in TE mode are limited in a range of about 0.5˜0.8dB substantially insensitive to all wavelengths varying from 1260 nm to1340 nm.
 13. The polarization splitter-rotator of claim 11, wherein thesecond width of the 2×2 MMI coupler waveguide is about 3 μm which isgreater than all widths of the rib structure waveguide, the planarwaveguide, and the phase-shifter.
 14. The polarization splitter-rotatorof claim 1, comprising a total length between 40 μm and 100 μm.
 15. Apolarization splitter-rotator for broadband operation comprising: a ribstructure waveguide with symmetrically tapered shapes including atop-layer overlying a bottom-layer extended in the lengthwise directionthrough a first length from an input port to a joint plane and a secondlength shorter than the first length from the joint plane to the firstcross-section plane, the top-layer being characterized with increasingwidths through the first length from a first common width at the inputport to a top width at the joint plane and further with increasingwidths through the second length from the top width to a second commonwidth at the first cross-section plane, the bottom-layer beingcharacterized with varying widths through the first length from thefirst common width at the input port to a bottom width at the jointplane and further through the second length from the bottom width to asecond common width; a planar waveguide extended further in thelengthwise direction from the first cross-section plane to a secondcross-section plane having a first port and a second port separated fromeach other, and configured to split light in C-band into the input portevenly to a first wave at the first port and a second wave at the secondport; a phase-shifter comprising a first waveguide arm connected thefirst port to a third port of a third cross-section plane and a secondwaveguide arm connected the second port to a fourth port of the thirdcross-section plane, the first waveguide arm being configured to keepthe first wave at the third port in-phase relative to that at the firstport, the second waveguide arm being configured to add a phase shift tothe second wave at the fourth port relative to that at the second port;and a 2×2 Multimode Interference (MMI) coupler extended further in thelengthwise direction from the third cross-section plane to an outputplane having a first output port and a second output port disposedrespectively in bar position relative to the third port and the fourthport, and configured to separately output a first output light in TEmode substantially originated from the light into the input port in TEmode and a second output light in TE mode substantially originated fromthe light into the input port in TM mode.
 16. The polarizationsplitter-rotator of claim 15, wherein the rib structure waveguide, theplanar waveguide, the phase-shifter, and 2×2 MMI coupler comprise amonolithic planar body patterned from a silicon layer of asilicon-on-insulator (SOI) substrate.
 17. The polarizationsplitter-rotator of claim 16, wherein the silicon layer comprises acommon height of proximately 220 nm.
 18. The polarizationsplitter-rotator of claim 15, wherein the light in C-band into the inputport comprises any wavelength from 1525 nm to about 1565 nm, wherein thebottom width is no greater than 1.7 μm and the first length is nogreater than 16 μm.
 19. The polarization splitter-rotator of claim 15,wherein the planar waveguide is characterized with symmetrically taperededges extended in the lengthwise direction with increasing widths in athird length started from the second common width at the firstcross-section plane to a first maximum width then continuously withdecreasing widths in a fourth length, and further continuously withdecreasing widths in a fifth length ended at the second cross-sectionplane, the first port and the second port being separated from eachother by a gap no smaller than 0.2 μm.
 20. The polarizationsplitter-rotator of claim 15, wherein the first wave at the first portcomprises a first TE1 sub-mode originated from the light into the inputport with the TM mode and a first TE sub-mode originated from the lightinto the input port with the TE mode, the second wave at the second portcomprises a second TE1 sub-mode originated from the light with the TMmode and a second TE0 sub-mode originated from the light into the inputport with the TE mode, wherein the first wave with the first TE1sub-mode is a phase π behind the second wave with the second TE1sub-mode and the first wave with the first TE0 sub-mode is in-phase withthe second wave with the second TE0 sub-mode.
 21. The polarizationsplitter-rotator of claim 20, wherein the first waveguide arm of thephase shifter comprises a straight bar shape of a sixth length and afirst arm width, and the second waveguide arm of the phase shiftercomprises a straight bar shaped portion joined aside with a triangleshaped portion along the sixth length and a second arm width increasingfrom the first arm width to a second maximum width near a middleposition of the six length and decreasing again to the first arm width.22. The polarization splitter-rotator of claim 21, wherein the phaseshifter is configured to make the first arm width to be no greater than0.5 μm and the second maximum width to be no greater than twice of thefirst arm width for keeping the first wave at the third port in-phaserelative to that at the first port through the first waveguide arm andgenerating the phase shift of (π/2+nπ) for the second wave relative tothat at the second port through the second waveguide arm, wherein n isselected from 0 and
 1. 23. The polarization splitter-rotator of claim22, wherein the 2×2 MIMI coupler waveguide comprises a rectangular shapehaving a seventh length and a second width configured to couple thefirst wave with the first TE1 sub-mode at the third port with the secondwave with the second TE1 sub-mode with a phase difference of (3/2)π toproduce the first output light in TE0 mode at the first output port, andconfigured to couple the first wave with the first TE0 sub-mode at thethird port with the second wave with the second TE0 sub-mode with aphase difference of (½)π to produce the second output light in TE0 modeat the second output port, wherein n is selected to be 0 in aconfiguration of the phase shifter.
 24. The polarizationsplitter-rotator of claim 23, wherein the seventh length is about 3 to 4times larger than the second width for generating the first output lightin TE mode with a transmission loss less than 1.3 dB relative to thelight into the input port in TE mode with any wavelength from about 1525nm to about 1565 nm and generating the second output light in TE modewith a transmission loss less than 1.4 dB relative to the light into theinput port in TM mode with any wavelength from about 1525 nm to about1565 nm, wherein the second output light includes −21 dB or smallerportion of the light into the input port in TE mode and the first outputlight includes −19 dB or smaller portion of the light into the inputport in TM mode.
 25. The polarization splitter-rotator of claim 24,wherein the transmission loss of the first output light relative to thelight into the input port in TE mode and the transmission loss of thesecond output light relative to the light into the input port in TM modeare limited in a range of 0.3 dB substantially insensitive to allwavelengths varying from 1525 nm to 1565 nm.